Surface acoustic wave device

ABSTRACT

The surface acoustic wave device has the surface acoustic wave transducer, which consists of the positive electrode finger  102 , the negative electrode finger  204 , and the floating electrode  300 , which are formed on the surface of the langasite single crystal substrate, where the substrate orientation and the surface acoustic wave propagation direction are chosen so that it may have the natural unidirectional property. When the wavelength of the surface acoustic wave is λ, each above-mentioned electrode is formed along the surface acoustic wave propagation direction, so that the width of above-mentioned positive electrode finger and the negative electrode finger may be about λ/8, the distance g between each center of the positive electrode finger and the floating electrode may be 13/40λ≦g≦14/40λ, and the width W of the floating electrode may be 11/40λ≦W≦13/40λ.

TECHNICAL FIELD

This invention relates to a surface acoustic wave device used for amobile-communications instrument etc.

BACKGROUND OF THE INVENTION

Although mobile communications instruments, such as a cellular phone anda portable terminal, have been spread rapidly in recent years, a filterused for these terminals is required for characteristics, that is lowenergy loss, a wide frequency band, and a small size, etc. Atransmitting type surface acoustic wave (SAW) filter is in practicaluse, in which said filter has a single-phase unidirectional transduceras a device which fulfills these characteristics. In a single-phaseunidirectional filter, a phase difference between an excitation wave anda reflective wave becomes the same phase to enhance each other in afront direction (at a forward direction), and is negated each other inan opposite direction (at a reverse direction). Therefore, the surfaceacoustic wave is strongly driven only at the forward direction. By thisway, it is theoretically possible to realize a low loss filter less than1 dB by opposing the direction of the unidirectional property of atransmitting electrode and a receiving electrode.

As the technology of realizing the unidirectional transducer, EWC-SPUDTand DART-SPUDT, using the asymmetrical electrode structure, aredesigned. Besides these filters using the asymmetry of the electrodestructure, there is a natural unidirectional filter (NSPUDT: NaturalSingle Phase Unidirectional Transducer). The natural unidirectionalfilter realizes unidirectional property by using the asymmetry of asubstrate crystal. Therefore, unidirectional property is realizable withthe transducer having the structure called regular type inter digitaltransducer (IDT) structure, in which the multiple positive/negativeelectrode fingers, where both the electrode width and the electrodespacing are λ/4, have been arranged continuously and periodically.

Even if regular type IDT is formed on a ST-X quart crystal substrate,the surface acoustic wave generated by the excitation driving of saidIDT is spread on a ST-X quart crystal substrate for the two way of saidIDT, and cannot realize unidirectional property. That is to say, thenatural unidirectional property shows the characteristic of thesubstrate, in which the surface acoustic wave is excited strongly to theuni-direction, when the regular type IDT is formed on a piezoelectriccrystal substrate surface. In the surface acoustic wave transducer usingthis natural unidirectional substrate, since an anisotropy of thesubstrate itself is used, the forward directions of thetransmitting-side transducer and the receiving-side transducer, cannotbe opposed. If the unidirectional property cannot be opposed between thetransmitting electrode and the receiving electrode, it is impossible toproduce the low loss filter.

As the means to solve this problem, in Tokukai Hei 8-125484, the surfaceacoustic wave transducer is proposed as the electrode structure toreverse the direction of natural unidirectional property by Mr. Takeuchietc., in which said surface acoustic wave transducer consisted of thepositive and negative electrode fingers, which had the width of almostλ/8 and were arranged with the pitch of λ, and the floating electrodes,which had the width of 3/8λ and were arranged with the edge interval ofalmost λ/8 between said positive electrode fingers and negativeelectrode fingers.

The low loss filter which opposed the unidirectional property, isproposed by Mr. Takeuchi etc. in Tokukai Hei 8-204492, in which saidfilter used the electrode structure of reversing the direction ofnatural unidirectional property. In the electrode structure of reversingthe direction of natural unidirectional property proposed by Mr.Takeuchi etc., when the wavelength of the surface acoustic wave is λ,the positive electrode finger and the negative electrode finger arearranged with the pitch of λ, respectively, the distance between eachcenter of the positive electrode and the negative electrode is λ/2, thefloating electrode having the width of λ/4 is formed between thepositive electrode finger and the negative electrode finger, and thedistance between each edge of the positive electrode finger or thenegative electrode finger and the floating electrode, is formed so thatit may be λ/16. Moreover, at least, the floating electrode of adjoiningone pair is short-circuited (FIG. 5 in the above-mentioned patent), andit is also included when all floating electrodes are short-circuited(FIG. 6 in the above-mentioned patent).

The characteristic of the surface acoustic wave device depends on thecharacteristic of the piezoelectric crystal used as the substrate. It isimportant that an electromechanical coupling coefficient is large, andan frequency temperature characteristic is good, as the characteristicof this piezoelectric crystal. Now, langasite is attracted as thecrystal which satisfies simultaneously these two characteristics. Whenlangasite is described in (φ,θ,ψ) by Euler-angles viewing, the langasitebeing in the range of −5°≦φ≦5°, 135°≦145°, and 20°≦ψ≦30°, has theelectromechanical coupling coefficients being 0.3% -0.4%, and thefrequency temperature characteristic showing the secondary dependency,and the summit temperature existing near a room temperature. Anelectromechanical coupling coefficient is about 3 times of ST quartcrystal. Moreover, the secondary temperature coefficient in thefrequency temperature characteristics is about 2 times of quart crystal,and this value is very good. Therefore, langasite is expected to beapplied to the low loss surface acoustic wave filter.

Langasite single crystal which is in the above-mentioned ranges byEuler-angle viewing, has NSPUDT characteristic, and for realizing thelow-loss filter using this substrate, it is necessary that the electrodestructure where the direction of unidirectional property counters in thetransmitting electrode and receiving electrode, is constituted.Therefore, when regular type IDT is used for the transmitting electrode,wherein the multiple positive/negative electrode fingers, where both theelectrode width and the electrode spacing are λ/4, have been arrangedperiodically and continuously, the structure, in which unidirectionalproperty is inverted, has to be used in the receiving electrode.However, in the electrode structure proposed by Mr. Takeuchi etc., theoptimum unidirectional inversion cannot be realized on the langasitesubstrate, and the request for realizing of the low loss filter cannotbe answered.

This invention was made in view mentioned above, and aims to offer thesurface acoustic wave device, which is enable to comprise the much lowloss transmitting type surface acoustic wave (SAW) filter.

DISCLOSURE OF INVENTION

The invention described in the first aspect is the surface acoustic wavedevice having the surface acoustic wave transducer consisting of thepositive electrode finger, the negative electrode finger, and thefloating electrode arranged between these fingers, which are formed onthe langasite single crystal substrate in which the substrateorientation and surface acoustic wave propagation direction are selectedto have the natural unidirectional property. Moreover, in theabove-mentioned surface acoustic wave transducer, each above-mentionedelectrode is formed along the surface acoustic wave propagationdirection in order to reverse the national unidirectional property.

The invention described in the second aspect is characterized with thesurface acoustic wave device according to the first aspect, in whichabove-mentioned langasite single crystal substrate is in the range−5°≦φ≦50°, 13°≦θ≦14°, and 20°≦ψ≦30°, or is the equivalent orientation inthe crystallography, when the substrate orientation and the surfaceacoustic wave propagation direction are described in (φ, θ, ψ) by Eulerangle viewing.

The invention described in the third aspect is characterized with thesurface acoustic wave device according to the second aspect, in which,in the distance relationships between the above-mentioned positiveelectrode finger, the negative electrode finger, and the floatingelectrode, in the above-mentioned surface acoustic wave transducer, thewidths of the above-mentioned positive electrode finger and the negativeelectrode finger, are about λ/8, the distance between each center of thepositive electrode finger and the negative electrode finger, is about6/8λ, the distance g between each center of the positive-electrodefinger and the floating electrode, is 13/40λ≦g≦14/40λ, and width W ofthe floating electrode is 11/40λ≦W≦13/40λ, when the wavelength of thesurface acoustic wave is λ.

The invention described in the fourth aspect is characterized with thesurface acoustic wave device having the surface acoustic wave transducerconsisting of the positive electrode finger, the negative electrodefinger, and the floating electrode arranged the between these fingers,which are formed on the langasite single crystal substrate in which thesubstrate orientation and surface acoustic wave propagation directionare selected to have the natural unidirectional property, and saidfloating finger is formed so that said floating electrode finger of anadjoining pair may short-circuit ranging over the above-mentionednegative electrode finger. Moreover, in the above-mentioned surfaceacoustic wave transducer, each above-mentioned electrode is formed alongthe surface acoustic wave propagation direction in order to reverse thenational unidirectional property.

The invention described in the fifth aspect is the surface acoustic wavedevice according to the fourth aspect, wherein said langasite singlecrystal substrate is in the range of −5°≦φ≦5°, 135°≦θ≦145°, and20°≦ψ≦30°, when said single crystal substrate is described in (φ, θ, ψ)by Euler-angles viewing, or the equivalent orientation of this value.

The invention described in the second aspect is the surface acousticwave device according to the fifth aspect, wherein, in the relationshipof the distance between above-mentioned positive electrode finger,negative electrode finger and floating electrode finger, the distancebetween each center of said positive electrode finger and negativeelectrode finger is about λ/2, the width d of both electrode fingers isabout λ/4, the distance g between each center of above-mentionedpositive electrode finger and the electrode finger of above-mentionedfloating electrode, which is one of the pair electrode fingersconsisting of said floating electrode and is adjoining to said positiveelectrode finger in above-mentioned surface acoustic wave propagationdirection, is 48λ/240≦g≦56λ/240, so that above-mentioned floatingelectrode is formed unevenly in which each electrode finger of said onepair electrode fingers closes to the positive electrode finger or thenegative electrode finger located at the adjoining left side.

The invention described in the seventh aspect is the surface acousticwave device having the surface acoustic wave transducer, which is formedon the langasite single crystal substrate in which the substrateorientation and the surface acoustic wave propagation direction areselected to have the national unidirectional property, and have thepositive electrode finger, which is arranged in the cycle of thewavelength A of the surface acoustic wave, and the 1st and 2nd negativeelectrode fingers, which are arranged on the one side of said positiveelectrode finger. Moreover, in the above-mentioned surface acoustic wavetransducer, the each above-mentioned electrode is formed along thesurface acoustic wave propagation direction so that naturalunidirectional property may be reversed.

The invention described in the eighth aspect is the surface acousticwave device according to the seventh aspects wherein the above-mentionedlangasite single crystal substrate has the substrate orientation and thesurface acoustic wave direction which are in the range of−5°≦φ5°,135°≦θ≦145° and 20°≦ψ≦30, or the equivalent to this values, whenthe substrate orientation and the surface acoustic wave propagationdirection are described in (φ, θ, ψ) by Euler-angles viewing.

The invention described in the ninth aspect is the surface acoustic wavedevice according to the eighth aspect, wherein, in the positionrelationship of the 1 st and 2nd negative electrode fingers and thewidth of these electrode fingers, the width of the above-mentionedpositive electrode finger is about λ/8, the distance d1 from the centerof said positive electrode finger to the center of the 1st negativeelectrode finger, in which the electrode width W1 is in the range of18/80λ≦W1≦20/80λ, is 23/80λ≦d1≦25/80λ, and furthermore, the distance d2from the center of said positive electrode finger to the center of the2nd negative electrode finger, in which the electrode width W2 is in therange of 20/80≦W2≦26/80λ, is 54/80λ d2 ≦55/80A, when the wavelength ofthe surface acoustic wave is λ.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is a schematic showing the electrode structure of a regular typeIDT.

FIG. 2 is explanatory drawing showing the position relationship betweenthe excitation center and the reflective center, for realizingunidirectional property by the regular type IDT shown in FIG. 1.

FIG. 3 is a schematic showing IDT of the conventional TCS-RDT structure.

FIG. 4 is a schematic showing the electrode structure of IDT which isused in the surface acoustic wave device of the example of thisinvention.

FIG. 5 is the characteristic figure showing the electrode layerthickness dependency of the phase term of the mode coupling coefficientκ₁₂.

FIG. 6 is the characteristic figure showing the electrode layerthickness dependency of the phase term of the excitation coefficient ξ.

FIG. 7 is the characteristic figure showing the electrode layerthickness dependency of the phase difference (α−β) of the mode couplingcoefficient, and the excitation coefficient.

FIG. 8 is the characteristic figure showing the electrode layerthickness dependency of the positions of the excitation center and thereflective center, in IDT of the TCS-RDT structure.

FIG. 9 is the characteristic figure showing the electrode layerthickness dependency of the positions of the excitation center and thereflective center in IDT which is used in the surface acoustic wavedevice of the example of this invention.

FIG. 10 is a schematic showing the structure of the transmitting typesurface acoustic wave filter, which is applied with this invention.

FIG. 11 is the characteristic figure showing the frequencycharacteristic of the transmitting type surface acoustic wave filterwhich is applied with this invention, and the transmitting type surfaceacoustic wave filter which is used IDT of the TCS-RDT structure as thereceiving electrode.

FIG. 12 is the characteristic figure which is expanded near the transitregion of the filter in the frequency characteristic shown in FIG. 11.

FIG. 13 is a schematic showing IDT of the conventional EWD-RDTstructure.

FIG. 14 is a schematic showing the electrode structure of IDT which isused in the surface acoustic wave device of the example of thisinvention.

FIG. 15 is the characteristic figure showing the electrode layerthickness dependency of the phase difference (α−β) of the mode couplingcoefficient, and the excitation coefficient.

FIG. 16 is the characteristic figure showing the electrode layerthickness dependency of the position of the excitation center and thereflective center in IDT of the EWD-RDT structure.

FIG. 17 is the characteristic figure showing the electrode layerthickness dependency of the position of the excitation center and thereflective center in IDT which is used in the surface acoustic wavedevice of the example of this invention.

FIG. 18 is a schematic showing the structure of the transmitting typesurface acoustic wave filter which is applied with this invention.

FIG. 19 is the characteristic figure showing the frequencycharacteristic of the transmitting type surface acoustic wave filterwhich is applied with this invention, and the transmitting type surfaceacoustic wave filter which is used IDT of the EWD-RDT structure as thereceiving electrode.

FIG. 20 is the characteristic figure which is expanded near the transitregion of the filter in the frequency characteristic shown in FIG. 19.

FIG. 21 is the characteristic figure showing the dependency of the phasedifference (α−β) of the mode coupling coefficient and the excitationcoefficient, to the ratio g/λ, in which λ is the wavelength of thesurface acoustic wave and g is the distance between each center of thepositive electrode finger and the floating electrode, in the surfaceacoustic wave transducer.

FIG. 22 is a schematic showing the electrode structure of IDT which isused in the surface acoustic wave device of the example of thisinvention.

FIG. 23 is the characteristic figure showing the electrode layerthickness dependency of the phase difference (α−β) of the excitationcoefficient ξ and the mode coupling coefficient κ₁₂.

FIG. 24 is the characteristic figure showing the electrode layerthickness dependency of the standardized excitation coefficient.

FIG. 25 is the characteristic figure showing the electrode layerthickness dependency of the positions of the excitation center and thereflective center in IDT, which is used in the surface acoustic wavedevice in the example of this invention.

FIG. 26 is a schematic showing the structure of the transmitting typesurface acoustic wave filter, which is applied with this invention.

FIG. 27 is the characteristic figure showing the frequencycharacteristic of the transmitting type surface acoustic wave filterwhich applied with this invention, and the transmitted type surfaceacoustic wave filter which is used IDT of the TCS-RDT structure as thereceiving electrode.

THE BEST FORM FOR THIS INVENTION THE 1st EXAMPLE

Hereafter, the 1st example of this invention is explained in detail byreferring the drawings. First, the principle is explained by referringFIG. 1, wherein the natural unidirectional property is occurred, whenthe so-called regular type electrode (regular type IDT) is formed on thelangasite piezoelectricity substrate, on which the multiplepositive/negative electrode fingers, in which the electrode width andthe distance between each electrode are λ/4, are arranged continuouslyand periodically, and is driven excitation. The schematic figure of theregular type electrode is shown in FIG. 1. In this figure, this regulartype electrode consists of the positive electrode 1 and the negativeelectrode 2, and an electric fields are occurred between the positiveelectrode finger 1A, which constitutes positive electrode 1, and thenegative electrode fingers 2A and 2B, which constitutes the negativeelectrode 2 being arranged on both side of this positive electrodefinger 1A. At this time, the excitation center of the surface acousticwave, which is generated on the langasite piezoelectricity substrate byexciting with this electric field, is nearly the center A of thepositive electrode finger 1A.

Moreover, in this electrode structure, the electrode fingers which hasthe electrode width of λ/4 and are arranged periodically, becomes thesource of the reflex of the surface acoustic wave. Since the reflex isoriginated in the discontinuity of the acoustic impedance, the surfaceacoustic wave is reflected at the edge of each electrode finger.Although the surface acoustic wave is reflected at two places of theboth ends of the electrode fingers in this way, it can be thought thatsaid wave is reflected equivalently at the center of the electrodefinger. The phase of the reflected wave is changed at this time. Thisvariation is dependent on the kind and the cut surface of thepiezoelectric substrate, the surface acoustic wave propagationdirection, and the kind of electrode materials and its thickness. Forexample, when a ST cut X propagation quart crystal is used as thepiezoelectric substrate and A1 is used as the metallic material, thephase of the reflection wave is late with 90°, that is, the phasevariation is 90°.

On the other hand, when the regular type IDT is formed, in which thelangasite single crystal is used as the substrate, A1 is used as theelectrode material, the phase variation of the surface acoustic wavereflected with the electrode fingers, becomes −90+2α, wherein thelangasite single crystal is in the range of −5 ≦φ≦5, 135≦θ≦145 and20≦ψ≦30, or the equivalent orientation to this value in crystallography,when the substrate orientation and the surface acoustic wave propagationdirection are described in (φ,θ,ψ) by Euler-angles viewing as thepiezoelectric crystal. This 2α is thought to be the phase shift at thetime of the reflection, and if the reflective center is defined as thereflective center is shifted from the center of the electrode fingeronly with the amount being equivalent to this 2α, the shift δ of thereflective center, is shown with the formula (1).δ=(α/2π)λ  (1).When δ is positive, the reflective center is shifted to right side fromthe center of the electrode finger, and when δ is negative, thereflective center is shifted to left side.

When the shift of the reflective center and the center of the electrodefinger, is λ/8, the phase at the point A of the wave excited with thepositive electrode finger 1A, the wave reflected at the reflectivecenter B of the negative electrode finger 2A, adjoining said electrodefinger 1A, and the wave reflected at the edge C of said electrode finger1A, is considered by using the FIG. 1 and, as a result, the phase at thepoint A, where the wave is reflected in the path of A→B→A, is shown withthe formula (2).−2×3λ/8×2π/λ−π/2=−2π  (2)This phase is the same phase as the excitation wave. On the other hand,the phase at the point A, where the wave reflected in the path of →C→A,is shown in the formula (3).−2×λ/8×2π/λ−π/2=−π  (3)This phase is the reverse phase to the excitation wave. For this reason,the surface acoustic wave will be driven strongly to right side of FIG.1, and unidirectional property is realized.

From the above mentioned reason, as shown in FIG. 2, when the distancebetween the excitation center and the reflective center becomes to theformula (4), it becomes possible to realize unidirectional property ofthe direction from the excitation center to the reflective center.λ/8+nλ/2(n=0, 1, 2 . . . )  (4)That is to say, if the position of the excitation center and thereflective center can be specified, it can be concluded whether or notthe surface acoustic wave transducer has unidirectional property, whenthe periodic electrode structure. (IDT), in which the surface acousticwave can be excited, is formed on the arbitrary crystal. The position ofthis excitation center and the reflective center, is described by themode coupling parameter, when a mode coupling theory is used.

The mode coupling parameter consists of the self coupling coefficientκ₁₁, the mode coupling coefficient κ₁₂, excitation coefficient ξ, andelectrostatic capacitance C. Here, the mode coupling coefficient κ₁₂ isexpressed as the formula (5), and is equivalent to the shift of thereflective center from base level, and the amount of said shift isexpressed with formula (1).κ₁₂=|κ₁₂ |e ^(j2α)  (5)Moreover, excitation coefficient ξ, is formula (6) and it may beconsidered that the excitation center is at the place where is separatedonly with the formula (7) from base level.ξ=|ξ|e ^(jβ)  (6)γ=(β/2π)^(λ)  (7)Therefore, it may be in the relation of formula (8), between the phasesof the mode coupling coefficient κ₁₂ and excitation coefficient ξ, sothat the difference between the reflective center and the excitationcenter may fulfill the formula (4).α−β=π/4+nπ(n: 0, 1, 2 . . . )  (8)Here, the result analyzed from the mode coupling theory is shown,wherein the unidirectional reversion electrode structure (it is calledTCS-RDT: Tranduction Center Shift type Reversal of DirectivityTransducer structure), which is proposed by Mr. Takeuchi in Tokukai Hei8-125484, and the position of the excitation center and reflectivecenter in the surface acoustic wave device of the example of thisinvention, are analyzed. The sectional plane and the propagationdirection of the langasite substrate are describe here in (0°,140°,24°)by the Euler-angles viewing. Moreover, Al is used as the electrodematerial. TCS-RDT structure is shown in FIG. 3 and the electrodestructure of the surface acoustic wave device of the example of thisinvention, is shown in FIG. 4.

In FIG. 3, the electrode of the TCS-RDT structure consists of thepositive electrode 10, the negative electrode 20, and both the electrodewidths of the positive electrode fingers 12 and 14, which constitutesaid positive electrode 10, and the negative electrode fingers 22 and24, which constitute said negative electrode 20, are λ/8, and thedistance between each center of the positive electrode finger 12 and thenegative electrode finger 24, is 6λ/8. Moreover, the electrode width ofthe floating electrode 30, which is prepared between the positiveelectrode finger 12 and the negative electrode finger 24, is 3λ/8, andthe distance g between the centers of the positive electrode finger 12and the floating electrode 30, is 3λ/8.

On the other hand, the electrode of the surface acoustic wavetransducer, which is used in the surface acoustic wave device of theexample of this invention, consists of the positive electrode 100 andthe negative electrode 200, as shown in FIG. 4. Both of the electrodewidths of the positive electrode fingers 102 and 104, which constitutethe positive electrode 100, and the negative electrode fingers 202 and204, which constitute the negative electrode 200, are λ/8. In addition,the distance between each center of the positive electrode finger 102and the negative electrode finger 204, is 6λ/8. Moreover, the electrodewidth of the floating electrode 300, which is prepared between thepositive electrode finger 102 and the negative electrode finger 204, is11λ/40, and the distance g between centers of the positive electrodefinger 102 and the floating electrode 300, is 13λ/40.

Moreover, in FIG. 3 and FIG. 4, the base levels of the phase ofexcitation coefficient & and the mode coupling coefficient κ₁₂ are bothcenters of the negative electrode finger 24 and 204 having the width ofλ/8.

In addition, the electrode layer thickness dependency of the phase term2α, is shown in FIG. 5, wherein 2α is the phase term of the modecoupling coefficient κ₁₂ of the electrode structure of the surfaceacoustic wave transducer, which is used in the surface acoustic wavedevice of the form of the TCS-RDT structure and the example of thisinvention. Moreover, the electrode layer thickness dependency of phaseterm β, is shown in FIG. 6, wherein β is the phase term of theexcitation coefficient ξ of the TCS-RDT structure and the electrodestructure of this invention. The electrode layer thickness dependency ofthe phase difference (α−β) between the excitation coefficient ξ and themode coupling coefficient κ₁₂, is shown in FIG. 7. The meaning that codeof the phase difference (α−β) indicates negative, is that the directionof the unidirectional property of the TCS-RDT structure is the reversedirection to the direction of the natural unidirectional property. Fromthis result, it is said that in the TCS-RDT structure, when thestandardization electrode layer thickness H/λ (H is the electrodethickness) is changed between 0 and 0.05, the magnitude of the phasedifference (α−β) is varied to −30° from near 0°, and does not reach to−45, which is the angle which optimizes unidirectional property, asclearly shown from the formula (8). On the other hand, by using theelectrode structure of the example of this invention, when thestandardization thickness is about 0.013, it can be understand that thephase difference (α−β) is −45°, which optimizes unidirectional property.

On the basis of the result of FIG. 3 and FIG. 4, the electrode layerthickness dependency about the position of the excitation center and thereflective center, to the TCS-RDT structure (FIG. 3), is shown in FIG.8. The electrode layer thickness dependency about the position of theexcitation center and the reflective center, to the electrode structure(FIG. 4) of the surface acoustic wave transducer, which is used for thesurface acoustic wave device of the example of this invention, is shownin FIG. 9. In each figure of FIG. 8 and FIG. 9, the schematic of theelectrode structure, is shown in the upside, and the sectional drawingof the electrode structure is shown in the lower graph, in order toclarify the position relationship of the surface acoustic wavepropagation direction. Moreover, in these figures, the reflective centeris shown with O and the excitation center is shown with X.

As shown in FIG. 9, in the electrode structure of the surface acousticwave transducer, which is used in the surface acoustic wave device ofthe example of this invention, since the reflective center is existed inthe left side to the excitation center, and the distance between theboth centers is about λ/8, it can be understand that the direction ofunidirectional property is at the left side on this figure, andunidirectional property is reversed to the direction of naturalunidirectional property.

Moreover, about the distance relationship between the above-mentionedpositive electrode finger, the negative electrode finger, and thefloating electrode, in the surface acoustic wave transducer, when thewavelength of the surface acoustic wave is λ, the width of the abovementioned positive electrode finger and the negative electrode finger,is about λ/8, the distance between each center of both electrode fingersis about 6/8λ, and the distance g between each center of the positiveelectrode finger and the floating electrode, is 13/40λ≦g≦14/40λ. Inaddition, if the width W of the floating electrode is 11/40λ≦W≦13/40λ,the unidirectional property can be reversed to the direction of naturalunidirectional property.

Next, two kinds of the prototype of the transmitting surface acousticwave filter, which was constituted by using the electrode structure ofthe surface acoustic wave transducer used in the surface acoustic wavedevice of the example of this invention, were made, and the result ofthese properties examined is shown. The cut surface/propagationdirection of the used langasite substrate, is described in (0°, 140°,24°) by Euler-angles viewing. Moreover, A1 was used as the electrodematerial. The structure of the 1st transmitting type surface acousticwave filter (it is described as filter #1), as the test sample, is shownin FIG. 10. In this figure, on the langasite substrate 300, the regulartype IDT310 as the transmitting electrode and IDT320 as the receivingelectrode are formed along the surface acoustic wave propagationdirection (the direction of +X). The regular type IDT310 consists of thepositive electrode 312 and the negative electrode 314, and the positiveelectrode finger 313 and the negative electrode finger 315, in whichboth width of the electrode and the distance between said electrodes areλ/4, are formed to be arranged continuously and periodically, to realizeunidirectional property by using NPUDT characteristics.

Moreover, IDT320 as the receiving electrode is used the electrodestructure of this invention, and consists of the positive electrode 322,the negative electrode 324, and the floating electrode 330. In here, theelectrode width of the positive electrode finger 323 and the negativeelectrode finger 325, is λ/8, and the distance between each center ofboth electrode fingers 323 and 325 is 6λ/8. In addition, the distance gbetween each center of the positive electrode fingers 323 and thefloating electrode 330, is 13λ/40, and the width W of the floatingelectrode 330 is 11λ/40. The structure of this receiving electrode isthe same structure as shown in FIG. 4.

In the 2nd transmitting type the surface acoustic wave filter as thetest sample 2 (it is described as the filter #2.), the same regular typeIDT as the transmitting type surface acoustic wave filter of theabove-mentioned 1st is used as the transmitting electrode, and the IDTof the TCS-RDT structure shown in FIG. 4, is used as the receivingelectrode. Both filters are arranged to be countered with theunidirectional property of the transmitting and receiving electrodes, asshown in FIG. 10. Moreover, on both endpoints of the langasite substrate300, the damper agent 340 is applied, in order to absorb the reflex ofthe surface acoustic wave at the edges. The cycle length λ of theelectrode fingers of filter #1 and #2, is 32.15 μm, and the A1 thicknessof the electrode is 500 nm (5000 A). The thinning weighting is given, inthe transmitting and receiving electrodes.

The measurement result of the frequency characteristic of the filter #1and the filter #2, is shown in FIG. 11 and FIG. 12. FIG. 12 is thefigure, which is expanded at near transit region of the filter in thefrequency characteristic shown in FIG. 11. FIG. 11 and 12 show that thepass-band insertion loss, the ripple in the band, and the retardationripple in the band, of this invention filter, are improved. Asspecifically shown in Table 1, as for the pass band insertion loss, thefilter #2 is −9.0 dB to the filter #1 being 8.0 dB, and as for theripple in the band, the filter #2 is 0.58 to the filter #1 being 0.24dB. Moreover, as for the retardation ripple in the band, the filter #2is 80.0 ns to the filter #1 being 69.5 ns.

THE 2nd EXAMPLE

Hereafter, the 2nd example of this invention, is explained in detail byreferring the drawings. First, the principle is explained by referringFIG. 1, in which the natural unidirectional property is occurred, whenthe so-called regular type electrode (regular type IDT) is formed on thelangasite piezoelectricity substrate, on which the multiplepositive/negative electrode fingers, in which the electrode width andthe distance between each electrode are λ/4, are arranged continuouslyand periodically, and are driven excitation. The schematic figure of theregular type electrode, is shown in FIG. 1. In this figure, this regulartype electrode consists of the positive electrode 1 and the negativeelectrode 2, and the electric field are occurred, between the positiveelectrode finger 1A which constitutes the positive electrode 1, and thenegative electrode fingers 2A and 2B, which constitute the negativeelectrode 2, which is arranged on both side of this positive electrodefinger 1A. At this time, the excitation center of the surface acousticwave, which is generated on the langasite piezoelectricity substrate, byexciting with this electric field, is nearly the center A of thepositive electrode finger 1A.

Moreover, in this electrode structure, the electrode fingers which haveλ/4 of the electrode width, and are arranged periodically, become thesource of the reflex of the surface acoustic wave. Since the reflex isoriginated in the discontinuity of the acoustic impedance, the surfaceacoustic wave is reflected at the edge of each electrode finger.Although, the surface acoustic wave is reflected at two places of bothends of the electrode finger in this way, it can be thought that saidwave is reflected equivalently at the center of the electrode finger.The phase of the reflected wave is changed at this time. This variationdepends on the kind and the cut surface of the piezoelectric substrate,the surface acoustic wave propagation direction, and the kind ofelectrode materials and its thickness. For example, when the ST cut Xpropagation quart crystal is used as the piezoelectric substrate, and A1is used as the metallic material, the phase of the reflection wave islate with 90°, that is, the phase variation is 90°.

On the other hand, when the regular type IDT is formed, in which thelangasite single crystal is used as the substrate, A1 is used as theelectrode material, the phase variation of the surface acoustic wavereflected with the electrode finger, becomes −90°+2α, wherein thelangasite single crystal, is within the range of −5°≦φ≦5°,135°≦θ≦145°and 20≦ψ≦30°, or the equivalent orientation to this value incrystallography, when the substrate orientation and the surface acousticwave propagation direction are described in (φ, θ, ψ) by Euler-anglesviewing as the piezoelectric crystal. This 2α is considered to be thephase shift at the time of the reflex, and if the reflective center isdefined, as the reflective center is shifted from the center of theelectrode finger only with the amount being equivalent to this 2α, theshift δ of the reflective center, is shown with the formula (1).δ=(α/2π)λ  (1)

When δ is positive, the reflective center is shifted to the right sidefrom the center of the electrode finger, and when δ is negative, thecenter is shifted to the left side.

When the shift of the reflective center and the electrode finger centeris λ/8, the phase at the point A of the wave driven with excitation atthe positive electrode finger 1A, the wave reflected at the reflectivecenter B of the negative electrode finger 2A adjoining said electrodefinger 1A, and the wave reflected at the edge C of the said electrodefinger 1A, are considered by using FIG. 1 and as a result, the phase atthe point A of the wave reflected in the path of A→B→A, is shown in theformula (2).−2×(3λ/8)×2π/λ−π/2=−2π  (2)This is the same phase as the excitation wave. On the other hand, thephase at the point A of the wave reflected in the path of A→C→A, isshown in the formula (3).−2(λ/8)×2π/λ−π/2=−2π  (3)This phase is the reverse phase to the excitation wave. For this reason,the surface acoustic wave will be excited at the right side of FIG. 1strongly, and unidirectional property is realized.

From the above mentioned reason, as shown in FIG. 2, when the distancebetween the excitation center and the reflective center, becomes to theformula (4), it becomes possible to realize unidirectional property ofthe direction from the excitation center to the reflective center.λ/8+nλ/2(n=0.1.2 . . . )  (4)That is, if the position of the excitation center and the reflectivecenter can be specified, it can be concluded whether or not the surfaceacoustic wave transducer has unidirectional property, when the cyclicelectrode structure (IDT), in which the surface acoustic wave can beexcited, is formed on the arbitrary crystal. The position of thisexcitation center and the reflective center, is described by the modecoupling parameter when the mode coupling theory is used.

The mode coupling parameter consists of the self coupling coefficient,the mode coupling coefficient, the excitation coefficient, and theelectrostatic capacitance C. Here, the mode coupling coefficient κ₁₂, isexpressed as the formula (5) and the phase of κ₁₂ is equivalent to theshift of the reflective center from base level, and said shift isexpressed with formula (1).κ₁₂=|κ₁₂ |e ^(12α)  (5)Moreover, the excitation coefficient ξ, is formula (6) and it may beconsidered that the excitation center is situated at the place where isseparated with only formula (7) from base level.ζ=|ζ|e ^(jβ)  (6)γ=(β/2π)λ  (7)Therefore, it may be in the relationship of formula (8), between thephase of the mode coupling coefficient κ₁₂, and excitation coefficients,so that the difference between the reflective center and the excitationcenter may fulfill formula (4).α−β=π/4+nπ(n=0.1.2 . . . )  (8)

Here, the result analyzed from the mode coupling theory, is shown,wherein the unidirectional reversion electrode structure (it ishereafter called an EWD-RDT structure), which is proposed by Mr.Takeuchi in the Tokukai hei 8-204492, and the position of the excitationcenter, and the reflective center in the surface acoustic wave device ofthe example of this invention, are analyzed. The sectional plane and thepropagation direction of the langasite substrate are described here in(0°, 140°, 24°) by Euler-angles viewing. Moreover, A1 is used as theelectrode material. EWD-RDT structure is shown in FIG. 13, and theelectrode structure of the surface acoustic wave device of the exampleof this invention, is shown in FIG. 14.

In FIG. 13, the electrode of the EWD-RDT structure consists of thepositive electrode 410, the negative electrode 420, and the floatingelectrode 430, and both of the electrode widths of the positiveelectrode fingers 412 and 414, which constitute the positive electrode410, and the negative electrode finger 422, which constitute thenegative electrode 420, are λ/8, and the distance between the eachcenter of the positive electrode finger 412 and the negative electrodefinger 422, is λ/2. Moreover, the electrode fingers which has the widthof λ/4, is between the positive electrode finger and the negativeelectrode finger. For example, the floating electrode 430 is formed,wherein the electrode fingers 432 and 434, which are the adjoining onepair, are short-circuited, and the distance between each edge of thepositive electrode finger 412, 414 or the negative electrode finger 422,and the floating electrode 430, is λ/16.

On the other hand, the electrode of the surface acoustic wavetransducer, which is used for the surface acoustic wave device of theexample of this invention, consists of the positive electrode 150, thenegative electrode 250, and the floating electrode 350, as shown in FIG.14. Both of the electrode widths of the positive electrode finger152,154, which constitute the positive electrode 150, and the negativeelectrode finger 252, which constitutes the negative electrode 250, areλ/16. The distance between the positive electrode finger 152 and thenegative electrode finger 252, is λ/2. Moreover, the floating electrode350 is formed, wherein one pair of electrode fingers 352 and 354 isformed to short-circuit with straddling over the negative electrodefinger 252 between the positive electrode finger 152 and the positiveelectrode finger 154, and the width of said electrode fingers 352 and354 is λ/4, and the distance g between the each center of the positiveelectrode finger 152 and the electrode finger 352, is 7λ/32, wherein thesaid electrode finger 352 is one of said pair electrodes 352 and 354,which constitutes said floating electrode 350, and adjoins the positiveelectrode finger 152 in the surface acoustic wave propagation direction.

Moreover, in FIG. 13 and FIG. 14, the base levels of the phase ofexcitation coefficient ξ and the mode coupling coefficient κ₁₂, are thecenters of the positive electrode finger 414 having λ/8 of width, andthe positive-electrode finger 154 having λ/16 of width.

Next, the electrode layer thickness dependency of the phase difference(α−β) between the excitation coefficient ξ and the mode couplingcoefficient, is shown in FIG. 15. The meaning, that the code of thephase difference (α−β) indicates negative, is that the direction of theunidirectional property of the EWD-RDT structure, is the reversedirection to the direction of the natural unidirectional property. Fromthis result, it is said that in the EWD-RDT structure, when thestandardization electrode layer thickness H/λ (H is the electrodethickness) is changed between 0 and 0.05, the magnitude of the phasedifference (α−β) is varied to −32° from near −37.5°, and does not reachto −45°, which is the optimum angle for unidirectional property, asclearly shown from the formula (8). On the other hand, by using theelectrode structure of the example of this invention, when thestandardization thickness is about 0.012, it can be understood that thephase difference (α−β) is −45° which optimizes unidirectional property.

On the basis of the result of FIG. 13 and FIG. 14, the electrode layerthickness dependency about the positions of the excitation center andthe reflective center, to the EWD-RDT structure (FIG. 13), is shown inFIG. 16. The electrode layer thickness dependency about the positions ofthe excitation center and the reflective center, to the electrodestructure (FIG. 14) of the surface acoustic wave transducer, which isused for the surface acoustic wave device of the example of thisinvention, is shown in FIG. 17. In each figure of FIG. 16 and FIG. 17,the schematic of the electrode structure, is shown in the upside, andthe sectional drawing of the electrode structure, is shown in the lowergraph, in order to clarity the position relationship of the propagationdirection of the surface acoustic wave. Moreover, in these figures, thereflective center is shown with O, and the excitation center is shownwith X.

As shown in FIG. 17, in the electrode structure of the surface acousticwave transducer, which is used for the surface acoustic wave device ofthe example of this invention, the reflective center is existed in theleft side to the excitation center, and since the distance between bothcenters is about λ/8, it can be understood that the direction of theunidirectional property is the left side on this figure, and is reversedto the direction of natural unidirectional property.

Moreover, the characteristics about g/λ dependency of the phasedifference (α−β) between the excitation coefficient ζ and the modecoupling coefficient is shown in FIG. 21, wherein the distance g is thedistance between each center of the positive electrode finger of thesurface acoustic wave transducer and the electrode finger of thefloating electrode, which is one of the pair electrodes constituting thefloating electrodes and adjoins said positive electrode finger in thesurface acoustic wave propagation direction, and λ is the wavelength ofthe surface acoustic wave. As shown in this figure, it can be understoodthat the phase difference (α−β) becomes −45°, which is the optimum anglefor reversing unidirectional property, when g/X is in the range of48/240≦g/λ≦56/240.

Moreover, wherein in the distance relationship between theabove-mentioned positive electrode finger, the negative electrodefinger, and the floating electrode, in the surface acoustic wavetransducer, when wavelength of the surface acoustic wave is λ, thedistance between each center of the above-mentioned positive electrodefinger and the negative electrode finger is about λ/2, the width d ofboth electrode finger is λ/20≦d≦λ/10, the width W of the electrodefinger of the above-mentioned floating electrode is about λ/4, and thedistance g between each center of the above-mentioned positive electrodefinger, and the center of the electrode finger of the above-mentionedfloating electrode is 48λ/240≦g≦56λ/240, wherein said electrode fingerof the above-mentioned floating electrode is one of the pair electrodefingers, which constitute the above-mentioned floating electrode, andadjoins said positive electrode finger in the above-mentioned surfaceacoustic wave propagation direction. In addition, in the above-mentionedfloating electrode, when each electrode finger of the above-mentionedpair is formed aside to be close to the positive electrode finger ornegative electrode finger, which is in the left side of said electrodefinger, the above-mentioned floating electrode can reverse theunidirectional property, to the direction of the natural unidirectionalproperty.

Next, the result of the evaluated characteristics is shown, wherein twokinds of the transmitting type surface acoustic wave filter, which was,constituted by using the electrode structure of the surface acousticwave transducer used for the surface acoustic wave device of the exampleof this invention, were made to test. The cut surface and thepropagation direction of the used langasite substrate, are shown in(0°,140°,24°) by Euler-angles viewing. Moreover, A1 was used as theelectrode material. The structure of the 1st transmitting type surfaceacoustic wave filter (where it is described as filter #1.) as thesample, is shown in FIG. 18. In this figure, on the langasite substrate500, regular type IDT510 as the transmitting electrode and IDT520 as thereceiving electrode, are formed along the propagation direction (+Xdirection) of the surface acoustic wave. The regular type IDT510consists of the positive electrode 512 and the negative electrode 514,and are formed so that the multiple positive-electrode fingers 513 andthe negative-electrode fingers 515, in which electrode width and thedistance between each electrode is λ/4, may be arranged continuously andperiodically, and has realized the unidirectional property using NSPUDTcharacteristics.

Moreover, IDT520 as the receiving electrode, uses the electrodestructure of this invention, and consists of the positive electrode 522,the negative electrode 524, and the floating electrode 530. Here, theelectrode widths of the positive electrode finger 523 and thenegative-electrode finger 525, are λ/16, the distance between bothelectrode fingers 523 and 525 is λ/2, the distance g between the eachcenter of the positive electrode finger 523 and the electrode finger 532which constitutes the adjoining floating electrode 530, is 7λ/32, andthe width W of the electrode finger of the floating electrode 530, isλ/4. The structure of this receiving electrode is the same as thestructure shown in FIG. 4.

The 2nd transmitting type surface acoustic wave filter (where it isdescribed as filter #2) as the sample, uses the same regular type IDT asthe above-mentioned 1st transmitting type surface acoustic wave filter,for the transmitting electrode, and uses the IDT of the EWD-RDTstructure shown in FIG. 13, for the receiving electrode. Both filtersare arranged so that the unidirectional property of the transmitting andreceiving electrodes may be countered, as shown in FIG. 18. Moreover,the damper agent 540 for absorbing the reflex of the surface acousticwave at the edge, is applied to the ends of the langasite substrate 500.The periodic length λ of the electrode finger of the filter # 1 and # 2,is 32.15 μm, the thickness of electrode A1 is 300 nm (3000 A). Thethinning weighting is given to the transmitting and receivingelectrodes.

The measurement results of the frequency characteristics of filter # 1and filter # 2, are shown in FIG. 19 and 20. FIG. 20 is the magnifiedfigure near the passing band of the filter in the frequencycharacteristics shown in FIG. 19. FIG. 19 and 20 show that the passingband insertion loss and the ripple in the band, of this invention, areimproved. Specifically, as shown in Table 2, as for the passing bandinsertion loss , the filter # 2 is −10.68 dB to the filter # 1 being−8.84 dB, and as for the ripple in the band, the filter # 2 is 0.46 dBto the filter poi being 0.33 dB.

THE 3rd EXAMPLE

Hereafter, the 3rd example of this invention, is explained in detail byreferring the drawing. First, the principle is explained by referringFIG. 1, wherein the natural unidirectional property is occurred, whenthe so-called regular type electrode (regular type IDT) is formed on thelangasite piezoelectricity substrate, on which the multiplepositive/negative electrode fingers which have λ/4 of the electrodewidth and the distance between each electrode, are arranged continuouslyand periodically, and is driven excitation. The schematic figure of theregular type electrode is shown in FIG. 1. In this figure, this regulartype electrode consists of the positive electrode 1 and the negativeelectrode 2, and the electric field is occurred between the positiveelectrode finger 1A which constitutes the positive electrode 1, and thenegative electrode finger 2A and 2B which constitutes the negativeelectrode 2, and are arranged on both side of this positive electrodefinger 1A. At this time, the excitation center of the surface acousticwave, which is generated on the langasite piezoelectricity substrate, bybeing excited with this electric field, is the almost center A ofpositive electrode finger 1A.

Moreover, in this electrode structure, the periodically arrangedelectrode finger having the electrode width λ/4, is the source of thereflex of the surface acoustic wave. Since the reflex is originated inthe discontinuity of the acoustic impedance, the surface acoustic waveis reflected at the edge of each electrode finger. Although the surfaceacoustic wave reflects at two places of both ends of the electrodefinger in this way, it can be thought that the said surface acousticwave reflects in equivalently at the center of the electrode finger. Thephase of the reflected wave is changed at this time. This variation isdepended on the kind of piezoelectric substrate and its cut plane, thepropagation direction of the surface acoustic wave, and the electrodematerial and its thickness. For example, when the ST cut X propagationquart crystal is used as the piezoelectric substrate, and A1 is used asthe metal material, the phase of the reflective wave is late with 90°,that is, the variation of phase is 90°.

On the other hand, when the langasite single crystal, is used as thesubstrate, in which langasite is in the range of −5°≦φ≦5°, 135°≦θ≦145°and 20°≦ψ30°, or is the equivalent orientation to this incrystallography, when the substrate orientation and the surface acousticwave propagation direction are shown by the Euler-angles viewing(φ,θ,ψ), as the piezoelectric crystal, and the regular type IDT isformed by using A1 as the electrode material on said langasitesubstrate, the variation of phase of the surface acoustic wave, which isreflected by the electrode finger, is 90+2α. When this 2α is consideredto be the phase shift at the time of the reflex, the reflective centeris defined, as the reflective center is shifted from the center of theelectrode finger only with the equivalent amount to this 2α.

Then, the shift δ of the reflective center is shown as the formula (1)δ=(α/2π)λ  (1).When δ is positive, the reflective center is shifted from the center ofthe electrode finger to the right side, and when δ is negative, thereflective center is shifted to the left side.

When the shift between the reflective center and the center of theelectrode finger, is λ/8, the phase at point A, of the wave, which isexcited at the positive electrode finger 1A, and the wave, which isreflected in each reflective center B and C of the negative electrodefinger 2A and 2B, is considered using the FIG. 1. First, the phase at Apoint of the wave, which is reflected in the path of A→B→A, is shown asthe formula (2)−2×3λ/8×2π/λ−π/2=−2π  (2)This reflective wave is the same phase as the excitation wave. On theother hand, the phase at A point of the wave, which is reflected in thepath of A→C→A, is shown as the formula (3)−2×λ/8×2π/λ−π/2=−π  (3)This reflective wave is the phase contrary to the excitation wave. Fromthis reason, since the surface acoustic wave will be excited stronglyrightward of FIG. 1, the unidirectional property is realized.

From the above thing, as shown in FIG. 2, when the distance of theexcited center and the reflective center, is fulfilled the relation ofthe formula (4), it becomes possible that the unidirectional property isrealized for direction of the excitation center to the reflectivecenter.λ/8+nλ/2(n=0,1,2, . . . )  (4)That is, when the periodic electrode structure (IDT), in which thesurface acoustic wave can be excited, is formed in the arbitrarycrystal, if the location of the excited center and the reflective centercan be specified, it can be decided whether the surface acoustic wavetransducer has the unidirectional property, or not. The location of thisexcited center and the reflective center, are described by the modecoupling parameter, at the time when the mode coupling theory is used.

The mode coupling parameter consists of the self bonding coefficientκ₁₁, the mode coupling coefficient κ₁₂, the excitation coefficient ζ,and the electrostatic capacitance C. Here, the mode coupling coefficientκ₁₂, is expressed with the formula (5).κ₁₂=|κhd 12 |e ^(j2α)  (5)The phase of the mode coupling coefficient κ₁₂ is equivalent to theshift of the reflective center from the base plane, and the amount ofthis shift is expressed with formula (1). Moreover, the excitationcoefficient ζ is shown with the formula (6).ζ=|ζ|e ^(jβ)  (6)The excitation center may be situated at the place where is onlyseparated with the amount of the formula (7) from the base plane.γ=(β/2π)λ  (7)Therefore, in order for the difference between the reflective center andthe excitation center to fulfill the formula (4), there may be requiredthe relationship of the formula (8) between the phases of the modecoupling coefficient κ₁₂ and excited coefficient ζ.α−β=π/4+nπ(n=0,1,2 . . . )  (8)Here, the result analyzed from the mode coupling theory, is shown,wherein the location of the excitation center and the reflective center;about the unidirectional property inversion electrode structure (it iscalled the TCS-RDT: Tranduction Center Shift type Reversal ofDirectivity Transducer structure), which is proposed by Mr. Takeuchi inthe Tokukai-Hei 8-125484, and about the electrode structure of thesurface acoustic wave device of the example of this invention, isanalyzed. The cut surface and the propagation direction of the langasitesubstrate, is shown (0°,140°,24°) by the Euler-angles viewing. Moreover,A1 is used as the electrode material. The TCS-RDT structure is shown inFIG. 3, and the electrode structure of the surface acoustic wave deviceof the example of this invention, is shown in FIG. 22.

In the FIG. 3, the electrode of the TCS-RDT structure consists of thepositive electrode 10 and the negative electrode 20, and when wavelengthof the surface acoustic wave, is λ, the electrode widths of both thepositive electrode fingers 12 and 14 and the negative electrode fingers22 and 24, are λ/8, in which the positive electrode fingers 12 and 14,constitutes the positive electrode 10, and the negative-electrodefingers 22 and 24, constitutes the negative electrode 20. In addition,the distance between each center of the positive electrode finger 12 andthe negative electrode finger 24 is 6λ/8. Moreover, the electrode widthof the floating electrode 30, which is prepared between the positiveelectrode finger 12 and the negative electrode finger 24, is 3λ/8, andthe distance g between each center of the positive-electrode finger 12and the floating electrode 30, is 3λ/8.

On the other hand, the electrode of the surface acoustic wavetransducer, which is used for the surface acoustic wave device of theexample of this invention, consists of the positive electrode 160 andthe negative electrode 260, as shown in FIG. 22, and when wavelength ofthe surface acoustic wave, is A, the widths of the positive electrodefinger 162 and 164, which constitute the positive electrode 160, areabout λ/8. In addition, the 1st negative electrode finger 262 and the2nd negative electrode finger 264, are prepared in one side (in FIG. 22,right-hand side) of one positive electrode finger 162. Here, thedistance d1, from the center of the positive electrode finger 162 to thecenter of the negative electrode finger 262, in which the electrodewidth W1 is in the range of 18/80λ≦W1≦20/80λ, is 23/80λ≦d1 25/80λ.Moreover, the distance d2 from the center of the positive electrodefinger 162 to the center of the negative electrode finger 264, in whichthe electrode width W2 is in the range of 20/80λ≦W2≦26/80λ, is54/80λ≦d2≦55/80λ.

The dependency of the electrode layer thickness of the phase difference(α−β) of the excitation coefficient ζ and the mode coupling coefficientκ₁₂, in the electrode structure of the surface acoustic wave transducer,which is used in the surface acoustic wave device of the TCS-RDTstructure and the example of this invention, is shown in FIG. 23. In theelectrode structure parameter of this receiving side electrode, thewidth of the 1st positive electrode finger of is about λ/8, and thedistance d1 between each center of the 1st negative electrode finger, inwhich the electrode width W1 is 20/80λ, and the 1st positive electrodefinger, is 23/80λ. Furthermore, the distance d2 between each center ofthe 2nd negative electrode finger, in which the electrode width W2 is26/80λ, and the 1st positive electrode finger, is 54/80λ.

From this result, in the TCS-RDT structure, the standardizationelectrode layer thickness H/λ (H is the electrode thickness), is changedbetween 0 and 0.05, and the magnitude of (α−β), is changed between −30°and near 0°, so it does not reach to −45°, which optimizes theunidirectional property, clearly from the formula (8). On the otherhand, by using the electrode structure of the example of this invention,when the standardization thickness is between about 0.01 and 0.05, itcan be understood that the value of the phase difference (α−β) becomes−45°, which optimizes the unidirectional property.

Moreover, the dependency of the electrode layer thickness ofstandardization excitation coefficient ζ·λ/2√{square root over ()}(ψ·C), in the surface acoustic wave device electrode structure of theexample of this invention, and the TCS-RDT structure, is shown in FIG.24. In the electrode structure of the surface acoustic wave device ofthe example of this invention, the magnitude of the standardizationexcitation coefficient increases about 10%, as compared with the TCS-RTDstructure. Since the excitation coefficient is equivalently to theconversion efficiency of the electric/sound conversion, if a big valueof the excitation coefficient is obtained, it is enable to produce thelow loss device.

The dependency of the electrode layer thickness about the location ofthe excitation center and the reflective center, to the electrodestructure of the surface acoustic wave device of the example of thisinvention, is shown in FIG. 25. In FIG. 25, the schematic of theelectrode structure of the surface acoustic wave device shown in FIG.22, is shown in the upper part, and, in the lower graph, the sectionaldrawing of the electrode structure of the above mentioned surfaceacoustic wave device, is shown corresponding to the floor plan, in orderto clarify the location relationship of the propagation direction of thesurface acoustic wave. Moreover, in these figures, O shows thereflective center and X shows the excited center.

As shown in this figure, in the electrode structure of the surfaceacoustic wave device of the example of this invention, it is understoodthat the reflective center is existed on the left side to the excitationcenter and, since the difference of both distances is about λ/8, thedirection of the unidirectional property is the left side of thisfigure, and the unidirectional property is reversed to the direction ofthe natural unidirectional property.

Next, two kinds of the transmitting type surface-acoustic wave filter,which are constituted by using the electrode structure of the surfaceacoustic wave transducer used for the surface acoustic wave device ofthe example of this invention, are made to test, and the result of theevaluated characteristics is shown. The cut surface and the propagationdirection of the used langasite substrate, is (0°,140°,25°) byEuler-angles viewing. Moreover, A1 was used as the electrode material.The structure of the 1st transmitting type surface acoustic wave filter(where it is described as filter # 1.) as the sample, is shown in FIG.26. In this figure, on the langasite substrate 600, the regular typeIDT610 as the transmitting electrode, and IDT620 as the receivingelectrode, are formed along the propagation direction (the direction of+X) of the surface acoustic wave. The regular type IDT610 consists ofthe positive electrode 612 and the negative electrode 614, and is formedso that the multiple positive electrode fingers 613 and the negativeelectrode fingers 615, with which both electrode width and electrodespacing is λ/4, may be arranged continuously and periodically, and hasrealized the unidirectional property by using NPUDT characteristics.

Moreover, IDT620 as the receiving electrode, uses the electrodestructure of the surface acoustic wave device of the example of thisinvention, and consists of the positive electrode 622 and the negativeelectrode 624. In the electrode structure parameters of this receivingside electrode, the width of the positive electrode finger 623 is aboutλ/8, and the distance d1 from the center of this positive electrodefinger 623 to the center of the 1st negative electrode finger 625, inwhich the electrode width W1 is 20/80λ, is 23/80λ. Furthermore, thedistance d2 from the center of the 2nd negative electrode finger 624, inwhich the electrode width W2 is 26/80λ, to the center of the 1stpositive-electrode finger 623, is 54/80λ.

On the other hand, the 2nd transmitting type surface acoustic wavefilter (where it is described as filter #2.) as the sample, used thesame regular type IDT as the transmitting type surface acoustic wavefilter of the above-mentioned 1st transmitting type surface acousticwave filter, for the transmitting electrode, and used IDT of the TCS-RDTstructure shown in FIG. 3, for the receiving electrode. Both filters arearranged so that the unidirectional property of the transmitting andreceiving electrodes may counter, as shown in FIG. 26.

Moreover, the damper agent 640 for absorbing the reflex of the surfaceacoustic wave at both edges is applied on both ends of the langasitesubstrate 600. The periodic length A of the electrode finger of thefilter #1 and #2, is 32.15 μm, and the thickness of A1 electrode of thefilter #1 and #2, is 500 nm (5000 A). The thinning weighting is given tothe transmitting and receiving electrodes.

The measurement result of the frequency characteristics of the filter #1and the filter #2, is shown in FIG. 27. FIG. 27 shows that the passingband minimum insertion loss, the ripple in the band, and the retardationripple in the band, of this invention filter, are improved. Asspecifically shown in Table 3, as for the passing band minimum insertionloss, the filter #2 is −9.0 dB to the filter #1 being −7.8 dB, and asfor the ripple in the band, the filter #2 is 0.58 dB to the filter #1being 0.21 dB. Moreover, as for the retardation ripple in the band, thefilter #2 is 80.0 nsec to the filter #1 being 67.3 nsec.

Availability on Industry

According to this invention, the surface acoustic wave device has thesurface acoustic wave transducer, which consists of the positiveelectrode finger, the negative electrode finger, and the floatingelectrode arranged between these fingers, which are formed on thelangasite single crystal substrate surface, in which the substrateorientation and the surface acoustic wave propagation direction werechosen, so that it might have the natural unidirectional property. Sincethe above-mentioned surface acoustic wave transducer is formed with eachabove-mentioned electrodes along the propagation direction of thesurface acoustic wave, so that the natural unidirectional property mightbe reversed, the parameter for specifying the electrode structure, itcan be possible to make the transmitting type surface acoustic wavefilter having low loss by choosing suitable parameters for specifyingthe electrode structure. That is to say, the width of theabove-mentioned positive electrode finger and the negative electrodefinger, the distance between each center of the above-mentioned positiveelectrode finger and the negative electrode finger, the distance betweeneach center of the above-mentioned positive electrode finger and thefloating electrode, and the width of the above-mentioned floatingelectrode, are chosen suitably.

According to this invention, the surface acoustic wave device has thesurface acoustic wave transducer, which consists of the positiveelectrode finger, the negative electrode finger, and the floatingelectrode arranged between these fingers, which are formed on thesurface of the langasite single crystal substrate, in which thesubstrate orientation and the surface acoustic wave propagationdirection were chosen, so that it might have the natural unidirectionalproperty, wherein this floating electrode was formed so that theelectrode finger of the adjoining couple might short-circuit rangingover the above-mentioned negative electrode finger. Moreover, since theabove-mentioned surface acoustic wave transducer is formed with eachabove-mentioned electrode along the propagation direction of the surfaceacoustic wave so that the natural unidirectional property might bereversed, it can be possible to make the transmitting type surfaceacoustic wave filter by choosing the suitable parameters for specifyingthe electrode structure. That is to say, the width of theabove-mentioned positive electrode finger and the negative electrodefinger, the distance between each center of the above-mentioned positiveelectrode finger and the negative electrode finger, the distance betweeneach center of the above-mentioned positive electrode finger and thefloating electrode, and the width of the above-mentioned floatelectrode, are chosen suitably.

In addition, according to this invention, the surface acoustic wavedevice has the surface acoustic wave transducer, which consists of thepositive electrode finger, which is formed on the surface of thelangasite single crystal substrate and installed in the cycle of thewavelength λ of the surface acoustic wave, wherein the substrateorientation and the surface-acoustic wave propagation direction werechosen so that it may have the natural unidirectional property, and the1st negative electrode finger and the 2nd negative electrode finger,which are installed on one side of said positive electrode finger. Sincethe above mentioned surface acoustic wave transducer is formed with eachabove-mentioned electrode along the propagation direction of the surfaceacoustic wave so that the natural unidirectional property might bereversed, it can be possible to make the transmitting type surfaceacoustic wave filter by choosing the parameters suitably for specifyingthe electrode structure. That is to say, the widths of the above the 1stand the 2nd positive electrode fingers and the 1st and 2nd negativeelectrode fingers, the distance between each center of the 1st positiveelectrode finger and the 1st and 2nd negative electrode finger, ischosen suitably.

TABLE 1 Filter #1 Filter #2 This Invention TSC-RDT Pass-Band MinimumInsertion −8.0 −9.0 Loss (dB) Ripple in Band (dB) 0.24 0.58 RetardationRipple in Band 69.5 80.0 (nsec)

TABLE 2 Filter #1 Filter #2 This Invention EWD-RDT Pass-Band MinimumInsertion −8.84 −10.68 Loss (dB) Ripple in Band (dB) 0.33 0.46

TABLE 3 Filter #1 Filter #2 This Invention TSC-RDT Pass-Band MinimumInsertion −7.8 −9.0 Loss (dB) Ripple in Band (dB) 0.21 0.58 RetardationRipple in Band 67.3 80.0 (nsec)

1. A surface acoustic wave device comprising: a surface acoustic wavetransducer including a positive electrode finger, a negative electrodefinger, and a floating electrode arranged between the positive andnegative electrode fingers, which are formed on a langasite singlecrystal substrate, in which a substrate orientation and a surfaceacoustic wave propagation direction are selected to have a naturalunidirectional property, wherein the positive and negative electrodefingers and the floating electrode are formed along the propagationdirection of the surface acoustic wave to reverse the nationalunidirectional property, wherein the langasite single crystal substrateis in a range −5°≦φ≦50°, 13°≦θ≦14°, and 20°≦ψ≦30°, or is the equivalentorientation in crystallography, when the substrate orientation and thesurface acoustic wave propagation direction are described in (φ,θ,ψ) byEuler-angles viewing and wherein a width of the positive and negativeelectrode fingers is about λ/8, a distance between each center of thepositive and negative electrode fingers is about 6/8λ, a distance gbetween each center of the positive electrode finger and the floatingelectrode is 13/40λ≦g≦14/40λ, and a width W of the floating electrode is11/40λ≦W≦13/40λ, when λ is a wavelength of the surface acoustic wave. 2.A surface acoustic wave device comprising: a surface acoustic wavetransducer including a positive electrode finger, a negative electrodefinger, and a floating electrode arranged the between the positive andnegative electrode fingers, which are formed on a langasite singlecrystal substrate in which a substrate orientation and a surfaceacoustic wave propagation direction are selected to have a naturalunidirectional property, and said floating electrode includes a pair offloating electrode fingers short-circuited over the negative electrodefinger, wherein the positive and negative electrode fingers and thefloating electrode are formed along the surface acoustic wavepropagation direction to reverse the national unidirectional property,wherein the langasite single crystal substrate is in a range of−5°≦φ≦5°, 135°≦θ≦145°, and 20°≦ψ≦30°, or an equivalent orientation ofthe range, when said substrate orientation and the surface acoustic wavedirection are described in (φ,θ,ψ) by Euler-angles viewing, and whereina distance between each center of the positive and negative electrodefingers is about λ/2, a width d of the positive and negative electrodefingers is about λ/4, a distance g between each center of the positiveelectrode finger and a corresponding adjoining floating electrode fingeris 48λ/240≦g≦56λ/240, and the floating electrode has an uneven shape inwhich each electrode finger of the pair of floating electrode fingerscloses to an adjoining left side positive electrode finger or negativeelectrode finger.
 3. A surface acoustic wave device comprising: asurface acoustic wave transducer formed on a langasite single crystalsubstrate in which a substrate orientation and a surface acoustic wavepropagation direction are selected to have a national unidirectionalproperty, and including a positive electrode finger arranged in a cycleof the wavelength λ of the surface acoustic wave, and including firstand second negative electrode fingers arranged on one side of saidpositive electrode finger, wherein the positive and negative electrodefingers and the floating electrode are formed along the surface acousticwave propagation direction to reverse the natural unidirectionalproperty, wherein the langasite single crystal substrate is in a rangeof −5°≦φ≦5°, 135°≦θ≦145° and 20°≦ψ≦30, or an equivalent to the range,when the substrate orientation and the surface acoustic wave directionare described in (φ,θ,ψ) by Euler-angles viewing, and wherein a width ofthe positive electrode finger is about λ/8, a distance d1 from a centerof said positive electrode finger to a center of the first negativeelectrode finger, in which the negative electrode finger width W1 is ina range of 18/80λ≦W1≦20/80λ, is 23/80λ≦d1≦25/80λ, and a distance d2 fromthe center of said positive electrode finger to a center of the secondnegative electrode finger, in which the negative electrode finger widthW2 is in a range of 20/80λ≦W2≦26/80λ, is 54/80λ≦d2 ≦55/80 λ, when thewavelength of the surface acoustic wave is λ.